Deformation and metamorphism in the Hylas zone and adjacent parts of the eastern Piedmont in Virginia

Abstract

Structural and petrographic studies of the Hylas zone northwest of Richmond, Virginia, reveal that late Paleozoic ductile shearing produced mylonites and ultramylonites from pre-existing biotite gneiss, granite gneiss, and amphibolite in the Goochland complex and from Petersburg(?) granite. Rocks in the Goochland complex west of the Hylas zone underwent at least two deformations (D 1 , D 2 ) prior to ductile shearing in the Hylas zone (D 3 ). D 3 caused a pervasive mylonitic foliation (S c ) and a later widely spaced shear cleavage (S s ) to be formed in rocks in the Hylas zone. Brittle deformation in the form of high-angle faulting and locally intense fracturing (D 4 ) was superimposed on the Hylas zone about 220 m.y. ago and is probably equivalent to the Palisades disturbance of New England. D 4 was associated throughout the Piedmont with synchronous downwarp and dominantly continental sedimentation to form a series of parallel Triassic basins represented in the study area by the Richmond Basin. Northwest-oriented high-angle faults that apparently displace Triassic sedimentary rocks are interpreted to predate the Late Cretaceous. Regional prograde metamorphism of the Goochland complex to amphibolite facies (M 1 ) was inclusive of D 1 and D 2 , because structural elements that compose these deformational events consist primarily of oriented metamorphic minerals. Well-equilibrated microstructures in the gneisses suggest that the M 1 peak persisted past D 2 . M 1 is inferred to have occurred about 340 m.y. ago. D 3 was accompanied by retrograde metamorphism (m 2 ) to greenschist facies in the Hylas zone during late Paleozoic time. The presence of laumontite, quartz, and calcite in brecciated zones suggests that brittle deformation during D 4 occurred under zeolite facies conditions. Cenozoic reverse faulting 90 km along strike of the study area in the Brandywine area of the Coastal Plain in Maryland may indicate a continuation of the Hylas zone to the north. Superposed deformation and intermittent reactivation of zones of instability similar to that of the Hylas zone are reported in the Eastern Piedmont fault system, which extends from Alabama to Virginia. Current stress release beneath the Coastal Plain may be influenced by the instability of the Hylas zone and analogous fault systems throughout the Piedmont.